Method and Use of an Apparatus for Recovery of Metals or Metal Compounds

ABSTRACT

A method for recovery of metals or metal compounds from metal-bearing solid particles comprising volatile compounds comprises supplying the solid particles ( 50 ) through a flame ( 25 ) of a burner ( 20 ). The volatile compounds are then evaporated by means of heat from the flame ( 25 ) without melting the solid particles, thereby providing a raw material product ( 34 ). The raw material product ( 34 ) is then recovered. By feeding the solid particles directly to the flame, a compact and efficient plant is provided. The melting process can be controlled in a satisfying way, avoiding melting of the metal-bearing solid particles supplied through the burner. The use of a burner is also described.

FIELD OF INVENTION

The present invention relates generally to a method for calcining ofmetal-bearing solid particles or dust. The invention also relates to theuse of an apparatus for performing such a method.

BACKGROUND

Large amounts of environmentally troublesome industrial wastes aregenerated in different processes in metallurgic industry. Such wastesinclude but are not limited to electric arc furnace (EAF) steel dustsand foundry wastes as well as zinc-bearing feeds, including foundrydusts and sludges.

A prior art plant for metals recycling from steelmaking and foundrywastes is described in the article “Metals recycling from steelmakingand foundry wastes by Horsehead resource development” published in 1992Electric furnace conference proceedings, pages 145-157. This prior artplant is shown in FIG. 1. The plant disclosed in this article comprisesrotary kilns, which are arranged to heat the process materials andthereby provide a recycling process. This process includes two majorsteps. In a first step the starting material, such as a mixture of zincbearing feed and coal or coke is fed to a first kiln and the startingmaterial is divided into iron-rich material and dust. The dust, which iscollected in a filter, comprises zinc oxide and other compounds andelements, such as salts, cadmium and lead. These other compounds andelements are unwanted and the dust is fed in a second step to a secondkiln and is there divided into zinc oxide and dust comprising the othercompounds and elements, for example salts, cadmium and lead. This secondstep is referred to as the calcining step. By calcining is generallymeant the conversion of the physical or chemical properties of asubstance by the application of heat.

An alternative process route to the prior art plant described aboveconsists of liquid slag fuming as the first step and calcining in arotary kiln as the second step.

In both the first and the second step of the first described processroute and the in the second step of the alternative process route, hotair and possibly oxygen is added to the processes. Also, in a rotarykiln a large part of the heating energy is transferred to the materialfrom the lining of the rotary kiln, i.e., as indirect heating.

A major drawback with the described recycling process is the fact thatthe rotary kilns are large and cumbersome to operate. For example, arotary kiln described in the above mentioned article could be of alength of up to 100 meters and have a diameter of up to three meters. Itis appreciated that a plant including rotary kilns of this kind isexpensive to operate.

Another drawback is the fact that the process time is rather long; theprocessed material is transported by means of gravity as the kilnrotates. This relatively long process creates a bottleneck in the totalrecovery process.

A further drawback is the fact that toxic compounds if present, such asdioxins, are evaporated by the process heat without being broken down.This is due to the limited process temperature of the counter currentkiln based calcining process. Vaporized dioxins are then directed to thegas cleaning section of the plant or even the ambient atmosphere,thereby constituting a hazard to the environment.

It is thus appreciated that the handling and recovery of metal-bearingdusts and solid particles from such processes are often difficult andcostly problems. The pressure from both public authorities and customersto find new solutions is constantly increasing.

Up to now no feasible alternative to the use of rotary kilns has beenfound for the process of calcining metal-bearing dusts wherein therecovered metal is obtained in solid form, i.e., wherein the recoveredmetal is not melted. It is however very difficult to combine anacceptable metallurgical result with an energy efficient andenvironmentally safe kiln process. Furthermore, due to the inherent lowweight dust is easily carried away by the gas to be mixed with theexhaust from the furnace, thus lowering the yield and constituting anenvironmental hazard.

The international publication WO01/86011 A1 discloses a method forrecovery of metals, wherein metallic fines are supplied to a flame of aburner (20) and the fines are brought to melt and to agglomerate. Theagglomerated product is then recovered.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for recoveryof metals and metal compounds from metal-bearing solid particles inwhich the above-mentioned drawbacks with known techniques are avoided orat least mitigated. Another object is to provide a use of an apparatusfor recovery of metals and metal compounds from metal-bearing solidparticles.

The invention is based on the realization that metal-bearing solidparticles can be fed directly through the flame of an oxy-fuel burnerwithout melting the metal or metal compound to be recovered, i.e., theoriginal shape and the state of aggregate of the solid particles aremaintained. The use of a burner in combination with feeding the solidparticles directly into and through the burner flame makes it possibleto control the heating process in such a way that the solid particlesare left unmelted after having passed the burner flame.

According to a first aspect of the present invention there is provided amethod for recovering of metals and metal compounds as defined in claim1.

According to a second aspect of the present invention there is provideda use of an apparatus for recovering of metals and metal compounds asdefined in claim 11.

With the method according to the invention, the problems of prior artare overcome or at least mitigated.

By feeding the solid particles directly to the flame, a compact andefficient plant is provided. The heating process can be controlled in asatisfying way, avoiding melting of the metal-bearing solid particlessupplied through the burner.

In a preferred embodiment, by using an oxygen-enriched gas with aburner, the control of the process is further enhanced and the volume ofthe exhaust gas is minimized.

BRIEF DESCRIPTION OF DRAWINGS

The invention is now described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of a prior art plant for recovery ofmetals from metal-bearing dust;

FIG. 2 is a schematic diagram of an apparatus used with the inventivemethod;

FIG. 3 is a sectional view of a burner used with the method according tothe invention;

FIG. 4 is a cross-sectional view of the burner shown in FIG. 2; and

FIG. 5 shows an alternative embodiment of an apparatus used with theinventive method.

DETAILED DESCRIPTION OF THE INVENTION

In the following, a detailed description of the method and the apparatusaccording to the invention will be given. The expressions dust and solidparticles will be used alternately in this description. By theseexpressions are to be understood particles in the solid state and havingan overall diameter of approximately 5 millimeters or below.Agglomerated products, i.e., sintered or compacted aggregates of fines,are here not included.

A prior art plant and a method of operating it have already beendescribed with reference to FIG. 1. In the first kiln, the startingmaterial, such as a mixture of zinc bearing feed and coal or coke isdivided into iron-rich material and zinc rich but contaminated dust.

In the second kiln is the zinc rich dust calcined to a zinc rich productwith low contents of impurities.

Starting with FIG. 2, an overall diagram of a plant for recovery andupgrading of metals and metal compounds, generally designated 10, isshown. In this context, the term recovery is intended to encompassrecovery with or without any further treatment of the resulting rawmaterial. The plant 10 is essentially identical to the plant describedin the international patent publication WO 01/86011. The plant is builtaround a burner 20 installed in a sidewall of a furnace 30. The burneris a so-called oxy-fuel burner and is thus supplied with fuel, such asfuel oil, propane, natural gas, or butane through a first feeding line21 and with oxygen through a second feeding line 22. By oxygen is inthis context meant a gas with an O₂ content exceeding 21% and preferablyso-called technical oxygen having an O₂ content of 90-99.5%.

Metal-bearing dust or solid particles are supplied through a thirdfeeding line 23. From the following description, it is clear that theinventive method is applicable to electric arc furnace (EAF) steeldusts, Waelz kiln dusts, slag fuming furnace dusts, lead and copperplant dusts and foundry wastes as well as zinc-bearing feeds, includingfoundry dusts. Thus, both fines, normally meaning products resultingfrom crushing and sintering, and dust, normally meaning productscollected in filters, are possible raw material for use with theinventive method as well as other equivalent materials, such as powder.

The metal-bearing solid particles also contain unwanted compounds, suchas cadmium, lead, different salts, sodium chloride, potassium chloride,oxides, fluorides etc. It will be appreciated that the expressioncompounds are to include elements.

The burner 20 will be described in more detail below with reference toFIGS. 3 and 4.

The third feeding line 23 is also connected to a feeder, generallydesignated 40. The feeder 40 comprises a silo 42, into which solidparticles are fed. The solid particles are directed from the silo 42 toa pressure vessel 44, from which they are further directed to the thirdfeeding line 23 connected to the burner 20. By means of thisarrangement, a desired supply rate of solid particles to the burner 20is ensured.

In an alternative embodiment, the solid particles are intermixed in afluid acting as bearer, thus creating a slurry that is fed to the burner20.

In the bottom of the furnace there is gathered a charge 34 resultingfrom the material supplied to the furnace 30.

The burner 20 will now be described in detail with reference to FIGS. 3and 4. The burner 20 comprises a main portion 24, to which the threesupply lines 21-23 shown in FIG. 2 are connected. The portion 24 isprovided with an essentially circular cross-section, see FIG. 4, inwhich the configuration of the supply lines 21-23 appears in moredetail. Fuel is supplied through the first supply line 21 in the form ofsix equidistant pipes 21 a-f placed at a constant distance from thecenter axis of the main portion 24. Oxygen is supplied through anannular outer portion 22 and thus surrounds the fuel supplied throughthe pipes 21 a-f. Finally, solid particles are supplied through the pipe23, which is co-axially placed in the burner.

As already mentioned, the burner 20 is mounted in the sidewall of thefurnace 30. In the preferred embodiment, the burner can be tilted, i.e.,can be positioned in different angles relative to the horizontal and thevertical. The different orientations can be used for obtaining desiredcharacteristics for the calcining process.

In the following, the method for recovering metals and metal compoundswill be described in detail.

Initially, dust is supplied to the silo 42 of the feeder 40. The dustused in the described process are metal-bearing solid particles. Thesolid particles making up the dust normally have an overall diameter ofless than approximately 5 millimeters, and preferably less thanapproximately 1 millimeter.

The dust fall from the silo and into the pressure vessel 44, wherein thepressure is maintained by means of a gas also functioning as a carryinggas, such as compressed air, oxygen, nitrogen or argon. By means of thepressure in the pressure vessel 44, the dust is then carried to theoxy-fuel burner 20 at a rate, which is determined by the pressure levelin the vessel 44, the amount of solid particles in the silo 42 etc.

The operation of the oxy-fuel burner 20 is controlled by means of theamount of fuel and oxygen supplied through the first and second supplylines 21 and 22, respectively. The supply lines are connected to sourcesof fuel and oxygen (not shown), as is conventional.

The operation of the burner 20 will now be described in detail withreference to FIGS. 3 and 4. Dust is supplied through the central feedingpipe 23 at a rate that is controlled by the feeder. Fuel is supplied inthe six fuel feeding pipes 21 a-f, see FIG. 4, while an envelope ofoxygen is supplied through the annular feeding area 22. The oxy-fuelmixture results in a flame 25 having properties, such as length,temperature etc., that are controlled by the supply rate of fuel andoxygen. The higher oxygen content, the higher temperature, resulting ina theoretical flame temperature as high as 1900-2500° C. and a flamevelocity of 100 meters per second.

Thus, the dust is injected into the central portion of the flame 25. Asis seen from FIG. 3, the dust injected into and through the flame 25 isleft unmelted by the heat of the flame, i.e., the original shape andstate of aggregate of the solid particles are maintained. This is madepossible by the fact that the solid particles remain in the flame for avery short time and under strictly controlled conditions. For example,the solid particles can remain in the flame for less than one second andmore preferably less than one half second. Thus, the heating ofparticles can be regulated so that—despite the high flametemperature—the particles are not melted but calcining is obtained.

Basically, the process is controlled by the ratio of solid particlessupplied into the flame and the amount of fuel burnt. However, theheating process is controlled by means of several parameters, of whichcan be mentioned: temperature and velocity of the flame 25, energycontent or density of the injected solid particles, stochiometry, i.e.,the ratio oxidizing gas to added fuel, the oxygen content of theoxidizing gas, the supply rate of oxygen and added fuel, the rate ofinjection of the dust and their characteristics, the travel time of thesolid particles in the flame, and burner characteristics andconfiguration, such as tilting. Thus the heating of particles can beregulated so that the calcined solid particles, such as zinc oxideparticles, fall to the bottom of the furnace 30, wherein they are addedto the charge 34. The particles can then be used as raw material forfurther processing. For example, the zinc oxide can be used as rawmaterial for zinc.

In the preferred embodiment, the process is run stochiometrically orsub-stochiometrically.

The evaporated compounds leave the furnace 30 through one or moreexhaust outlets (not shown) and are taken care of in some convenientway. It is believed that the inventive method using relatively highflame temperature breaks down some unwanted toxic compounds, such asdioxin, thereby preventing them from entering the ambient atmosphere.

Preferred embodiments of the method and the use of an apparatusaccording to the invention have been described. The person skilled inthe art realizes that these can be varied within the scope of theappended claims. Thus, although an oxy-fuel burner 20 has been shown,other equivalent burners, such as plasma burners, can be used as long asdesired oxygen levels exceeding 21% are obtained. Also oxy-fuel burnersof other configurations than the disclosed one can be used, such as aburner with a different number of fuel pipes than six.

In the embodiment shown in FIG. 2, the burner is positioned in asidewall of a furnace. However, it is realized that other suitablepositions are possible, such as in the upper part of the furnace. Also,a configuration with more than one burner is also possible. In FIG. 5,yet an alternative embodiment is shown, wherein the burner 20 isprovided in one end of a rotary kiln 30′. In this way, an existing plantcan be retrofitted with a burner at the inlet end of the kiln, providinga co-current process instead of the prior art counter-current process.The solid particles are fed to the burner in the same way as in theembodiment described with reference to FIG. 2 but are transported awayby the rotation of the kiln 30′.

In the described embodiments, the solid particles are fed to the furnaceby means of a feeder. However, the particles supplied to the furnacecould also be free-flowing, carried by means of a feed gear etc.

A dry starting material has been shown in the figures. In the case thedust particles are intermixed in a liquid, such as water or sludge, asuitable feeding arrangement must be provided, comprising a feed screw,for example. Also, when arriving to the furnace, the wet part isvaporized by the high temperature of the flame, resulting in exhaustsrising through the furnace 30 and subsequently leaving through anexhaust outlet (not shown).

1. A method for recovery of metals or metal compounds from metal-bearingsolid particles comprising volatile compounds, the method comprising thefollowing steps: a) supplying the solid particles (50) through a flame(25) of a burner (20), b) bringing the volatile compounds to evaporateby means of heat from the flame (25) while maintaining the originalshape and state of aggregate of the solid particles, thereby providing araw material product (34), and c) recovering the raw material product(34).
 2. The method according to claim 1, wherein the burner (20) is anoxy-fuel burner.
 3. The method according to claim 1, wherein the burneris supplied with an oxidizing gas having an O₂ content of at least 21%O₂.
 4. The method according to claim 3, wherein the burner is suppliedwith an oxidizing gas having an O₂ content of at least 90% O₂.
 5. Themethod according to claim 1, wherein the solid particles (50) comprisezinc oxide.
 6. The method according to claim 1, wherein the volatileelements are elements from the group comprising cadmium, lead, differentsalts, sodium chloride, potassium chloride, oxides, and fluorides. 7.The method according to claim 1, wherein the solid particles (50) have adiameter of less than approximately 5 millimeters, and more preferably adiameter of less than approximately 1 millimeter.
 8. The methodaccording to claim 1, wherein the calcining process of step b) iscontrolled by means of at least some of the following parameters:temperature and velocity of the flame (25), energy content or density ofthe solid particles, the ratio oxidizing gas to added fuel of the burner(20), the oxygen content of the oxidizing gas, the supply rate of oxygenand added fuel, the oxygen content in exhaust gas, the supply rate ofthe solid particles and their characteristics, the travel time of thesolid particles in the flame, and burner characteristics andconfiguration, such as tilting.
 9. The method according to claim 1,wherein step a) comprises supplying the solid particles (50) essentiallyto the central portion of the flame (25).
 10. The method according toclaim 1, wherein the solid particles remain in the flame (25) for lessthan one second, and more preferably for less than one half second. 11.Use of an apparatus for recovery of metals and metal compounds frommetal-bearing solid particles comprising volatile compounds, theapparatus comprising: a burner (20) having a flame (25) during operationthereof, a feeder (40) connected to the burner for supplyingmetal-bearing solid particles comprising volatile compounds through theflame (25) of the burner (20), a device (21,22) for controlling theflame (25) of the burner (20) so as to evaporate the volatile compoundsby means of heat from the flame while maintaining the original shape andstate of aggregate of the solid particles, thereby providing rawmaterial product (34), and a device (30) for recovering the raw materialproduct (34).